The vascular network of the brain forms a biological barrier known as the blood-brain barrier (BBB). The BBB possesses a unique vascular phenotype that is induced by neighboring cells such as pericytes, astrocytes and neurons that together form the neurovascular unit (Abbott, N. J. and A. Friedman, Overview and introduction: The blood-brain barrier in health and disease. Epilepsia, 2012. 53: p. 1-6). This phenotype is best characterized as a combination of properties designed to maintain brain homeostasis, including tight paracellular junctions, a significant transporter repertoire, and a low basal level of pinocytosis, thereby rendering the BBB selectively permeable to required ions, nutrients and cells (Stutz, C., X. Zhang, and E. Shusta, Combinatorial Approaches for the Identification of Brain Drug Delivery Targets. Curr. Pharm. Des. 2013). While the BBB helps the brain to maintain the specific environment necessary for neuron function, it also prevents most small and large molecule therapeutics from gaining access to the brain (Jones, A. R. and Shusta, E. V., Blood-brain barrier transport of therapeutics via receptor-mediation; Pharm. Res., 2007. 24(9): p. 1759-1771). The BBB is therefore a major impediment to the treatment of central nervous system disease, and effective delivery strategies remain scarce.
One promising delivery method involves targeting known receptor-mediated transport systems with antibodies to mediate non-invasive drug delivery past the BBB. Two prominent examples of this approach are antibodies that target the transferrin and insulin receptors (Pardridge, W. M., Y. S. Kang, and J. L. Buciak, Transport of Human Recombinant Brain-derived Neurotrophic Factor (BDNF) Through the Rat Blood-Brain Barrier in vivo Using Vector Mediated Peptide Drug Delivery; Pharm. Res., 1994. 11(5): p. 738-746; Pardridge, W. M., Y. S. Kang, J. L. Buciak, and J. Yang, Human Insulin Receptor Monoclonal Antibody Undergoes High Affinity Binding to Human Brain Capillaries in vitro and Rapid Transcytosis Through the Blood-Brain Barrier in vivo in the Primate. Pharm. Res., 1995. 12(6): p. 807-816). While these systems allow for therapeutic amounts of drug to penetrate the BBB, they are inherently non-selective and relatively inefficient. The current lack of brain drug delivery systems with ideal selectivity and efficiency has motivated the search for new antibodies capable of targeting and/or transporting therapeutic payloads into the brain (Stutz, C., X. Zhang, and E. Shusta, Combinatorial Approaches for the Identification of Brain Drug Delivery Targets. Curr. Pharm. Des. 2013).
To address this problem, one useful approach that has been employed is antibody-based screening. Such screens have been used to identify BBB cell surface proteins that can mediate brain targeting and, in some cases, transport. For instance, large combinatorial antibody libraries have been screened against brain endothelial cells in various formats, in vitro or in vivo, to discover both antibody targeting molecules and cognate brain endothelial cell proteins. While the identified antibody-BBB antigen pairs look promising for circumventing the BBB, in some cases only a handful of new antibodies have been isolated (Muruganandam, A., J. Tanha, et al., Selection of Phage-displayed Llama Single-Domain Antibodies That Transmigrate Across Human Bloodbrain Barrier Endothelium; FASEB J., 2001. 15(14): p. 240). However, multiple genomic and proteomic studies support substantial differences in gene expression between the brain microvascular endothelium and the peripheral microvasculature, particularly in areas of transport and signaling between the brain and bloodstream (Li, J. Y., R. J. Boado, and W. M. Pardridge, Blood-brain Barrier Genomics, J. Cereb. Blood Flow Metab., 2001. 21(1): p. 61-68; Calabria, A. R. and E. V. Shusta, Blood-brain Barrier Genomics and Proteomics: Elucidating Phenotype, Identifying Disease Targets and Enabling Brain Drug Delivery, Drug Discov. Today, 2006. 11(17-18): p. 792-799; Daneman, R., L. Zhou, D. Agalliu, J. D. Cahoy, et al., The Mouse Blood-Brain Barrier Transcriptome: A New Resource for Understanding the Development and Function of Brain Endothelial Cells. PLoS One, 2010. 5(10)).
Needed in the art is an improved BBB-selective antibody.